I think it has been mentioned on the show that Kim is in her late thirties. I could believe early forties.Jodi looks about 45. Her face is lined and saggy. (I am 48 and I look younger.) Jill is definitely over 40. Kelly could be late 40s. Lisa looks around 50, but still lookin' good. Christie could be anywhere in her 30s. Granted, if Jodi and Jill (and Allison) lost weight, they might look much younger.
As for Allison, I remember at her funeral service, the tombstone or whatever said, "1964-2005" so no way is she 38 or 39! Her hormonal situation, with the cancer and then the hysterectomy, has made her look more like a menopausal or post-menopausal woman. A lot of cancer patients get puffy-looking.
As for the earlier housemates, I thought Jessica and Christina looked about right for their ages (though Christina could have passed for younger). I figured TJ was around Kim's age, but I don't remember anyone mentioning her age.